Nitrogen Fixation by Rhizobium Bacteria

Steps:

• Identify the nitrogen cycle process: Root nodule bacteria (Rhizobium) in leguminous plants fix atmospheric N2 into usable forms like ammonia.
• Locate the arrow in the diagram that shows N2 from the atmosphere entering the soil or plants via fixation.
• Match this to the choice depicting conversion of N2 gas to nitrates or ammonia in roots.
• Confirm by excluding arrows for other processes like denitrification or ammonification.

Why A is correct:

• Arrow A represents nitrogen fixation, where Rhizobium bacteria in root nodules convert atmospheric N2 to NH3, per the biological nitrogen fixation process.

Why the others are wrong:

• B shows denitrification, converting nitrates back to N2 gas by soil bacteria.
• C depicts nitrification, oxidizing ammonia to nitrates by Nitrosomonas and Nitrobacter.
• D illustrates ammonification, decomposing organic matter to release ammonia.

Final answer: A